Function is

blocksuite/framework/store/src/extension/selection/base.ts:39–43

(
    type: T
  )

  abstract equals(other: BaseSelection): boolean;

  is<T extends SelectionConstructor>(
    type: T
  ): this is T extends SelectionConstructor<infer U> ? U : never {
    return this.type === type.type;
  }

  abstract toJSON(): Record<string, unknown>;
}